Heat-sensitive copy-sheet and composition therefor



April 24, l

962 E. WINGERT 3,031,329

HEAT-SENSITIVE COPY-SHEET ANO COMPOSITION THEREFOR Filed Oct. 2e, 1959 @XM/'zing age/72j organ/'c reduc/'ng agem and per/m/ogenqed tte This invention relates to heat-sensitive copy-papers as employed in the thermographic repro-duction of type- Written correspondence and other graphic originals, and is particularly directed to improvements in the stability of certain visibly chemically reactive compositions as used in said copy-papers.

An important thermographic reproduction process, extensively employed in reproducing office correspondence and the like, involves pressing a heat-sensitive copy-sheet into heateconductive contact with a differentially radiation-absorptive graphic original which is subjected to brief intense irradiation, eg. with radiation rich in infrared. The heat-pattern produced at the original transfers to the copy-sheet, Where a corresponding visible change results. A direct reproduction of the original is obtained.

One form of heat-sensitive copy-sheet which has found extensive commercial utility in the process just described depends for its visible change on a heat-induced oxidation-reduction reaction, for example between a noble metal salt such as silver behenate and an organic reducing agent such as protocatechuic acid. The reaction results in the liberation of the Ifree metal, and produces a distinct visible change at the heated areas.

The mixtures of metal soaps and reducing agents employed in these heat-sensitive compositions remain visibly unchanged and fully reactive when stored under condi- -tions of normal room temperatures and humidities, either .prior to application or as thin heat-sensitive coatings on paper or other supporting media. At higher temper-atures or under other more drastic storage or use conditi-ons,` the reactants in some instances have been found to slowly react and darken. Thus, heat-sensitive copy-sheets prepared with silver behenate and protocateehuic acid as the sole reactants have shown significant darkening when stored for several hours at temperatures approaching 60 C. The same reactants in suspension in a solution of film-forming binder 4have similarly undergone appreciable reaction during `overnight storage of the uid suspension.

The present invention provides improved coa-ting compositions and coated sheet materials wherein undesired slow reaction of the metal soap and organic reducing agent in iiuid suspension or in thin lsolid iilm form at moderately elevated temperatures is significantly retarded or prevented. Surprisingly, diminishing the reactivity of the image-forming components under storage conditions by such means is found to have no observable eiect on image formation Iat the temperatures utilized in the thermographic reproduction process.

These signiiicant advantages are attained by incorporating with the mixture of metal soap and organic reducing agent a small but significant amount of a highly halogenated aromatic polycarboxylic acid or anhydride. Tetrachlorophthalic anhydride is `a preferred stabilizer material; tetrabromophthalic anhydride and tetrachlorophthalic acid are also effective. It Will be observed that these preferred stabilizers are perhalogenated phthalic acidic compounds. They do not act as reducing agents for the metal soap.

Example 1 A mixture of equal mol percent of silver behenate and behenc acid is prepared by precipitation from a mixture mi?. i

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of one mol of silver nitrate and two mols of sodium behenate, made from commercial behenic acid, in aqueous medium. The resulting water-insoluble precipitate is recovered on a filter and is washed and dried to produce a line powder. The powder fuses at about 135 C. and melts to a liquid at about 175 C. Silver behenate and behenic acid, prepared separately from the same sodium behenate, melt at about 220 C. and about 70 C. respectively.

. The powdery silver behenate-behenic acid mixture is intimately dispersed in -a solution of polystyrene resin by ball milling, the proportions being Parts by weight Silver behenate-behenic acid 8.8 Polystyrene (Dow 700) 2.2 Acetone 49.0 Heptane 40.0

Separately there is prepared a solution of reducing agents in polystyrene resin, having the formula Coating mixtures are prepared by mixing together in each case portions representing ten parts by Weight of the dispersion and thirteen parts of the solution. Prior to mixing, small amounts of tetrachlorophthalic anhydride are added to the por-tions of the solution, the' amounts being varied `from 0.1 to 1.0 part based on parts of the solution. A further mixture containing no tetrachlorophthalic anhydride is retained as a control.

The several mixtures are first tested for rate of deterioration in solution by measuring the relative transmission of light through a constant depth of the material. Light from a xed source is directed through the liquid to a photocell and the output of the cell measured on an appropriate meter. As first prepared, the control mixture gives a meter reading of 29. After three hours at room temperature, the same sample gives a meter reading of zero, and has deteriorated and darkened to a degree requiring it to be discarded. The mixtures containing tetrachlorophthalic anhydride give initial meter readings of 35-38. After three hours at room temperature, these samples give meter readings of 27-30, and are found to be useful in the preparation of fully effective white or light-colored heat-sensitive copy-sheets.

Portions of each of the several mixtures are also tested in the form of thin dry coatings on dense transparent paper. The mixtures are coated on map overlay tracing paper, using a coating bar at an oriiice of 2 mils, and the coatings dried in a current of air at normal room temperature. The sheets are placed in an oven at 60 C. for two hours and are then tested for discoloration with a light-meter, by comparison with the unheated original sheet. The meter measures the relative proportion of incident =light, from a standard source, which is absorbed -by the surface being tested, and is first adjusted to give a zero reading with the surface of the unheated sheet. The control sample shows a meter reading of 28, indicating extensive darkening. At 0.1 and 0.2 percent added anhydride, substantial darkening occurs, as indicated by a meter reading of 21-22. At 0.3 percent and higher, the anhydride effectively retards or prevents darkening, the meter readings being 3 at 0.3%, 51/2 at 0.4%, and decreasing to zero, i.e. total lack of observable darkening, at 1.0% of tetrachlorophthalic anhydride. =For commercially acceptable copy-sheets the meter reading obtained after two hours at 60 C. should be not greater A mixture of equimolar proportions of silver behenate and behenic acid, prepared as described in connection with Example 1, is dispersed in a solution of polystyrene by ball milling. The proportions of the silver `soap dispersion are:

Parts by weight Silver behenate mixture 8.8 Polystyrene 2.2 Acetone 56 Heptane 33 A binder solution is separately prepared by dissolving a styrene-isrobutylene copolymer resin in a solvent mixture, the components and proportions being:

Parts by weight Resin 7.5 Acetone 14.5 Heptane 51.8

A solution of reducing vagent and other components is also prepared, having the following composition:

Parts by weight Methyl gallate 1.5 Phthalic anhydride (optional) .25 Citric acid (optional) .25 Acetone 5.5 Tetrachlorophthalic anhydride 0.3

Portions of the three compositions are mixed together in the weight ratio of 51 :73.8:7.5, and the mixture is coated on a transparent paper carrier and dried at room temperature. The liquid mixture is stable during prolonged coating schedules in making commercial quantities of the coated sheet product. The initially essentially white sheet remains uncolored under normal office storage and use conditions for prolonged periods. When held for two hours at 60 C. the sheet undergoes a slight darkening but is still commercially acceptable.

Replacing the tetrachlorophthalic anhydride with 0.2 part of tetrabnomophthalic anhydride in the foregoing formula results in a sheet which is visibly darkened after two hours at 60 C. but is still useful where highest contrast is not required.

Elimination of the tetrachlorophthalic anhydride or equivalent from the copy-sheet formula, on the other hand, results in a sheet which darkens excessively in the heat test, particularly when carried out under conditions of high humidity. With the -acidic additive the improved stability is obtained regardless of humidity.

Example 3 A heat-sensitive copy-sheet prepared with a composition of generally the formulations of the preceding examples but based on the combination silver behenate: 1,4-dihydroxynaphthalene is initially white in color and is stable at normal storage temperatures and at low hurnidities but darkens appreciably in two hours at temperatures approaching 60 C. and more particularly under conditions of high humidity. The addition of small amounts of tetrachlorophthalic anhydride or tetrabromophthalic anhydride imparts a light buff color to the resulting copy-sheet, thereby reducing its commercial attractiveness although effectively preventing further color change in two hours at 60 C. and under high humidity. On the other hand, tetrachlorophthalic acid addedl to the basic formulation in the same amount provides equally effective improvement in sheet stability without any visible alteration in the initial appearance of the white heatsensitive coating.

Normally solid noble metal salts of organic acids are in general useful in place of the silver behenate of the foregoing example. Water-insoluble salts, particularly of the long-chain acids, are preferred a-s imparting resistance to the degradative effects of high humidity and fingerprinting. A partial list of salts which have been found useful includes silver stearate, gold stearate, and silver salts of oleic, lauric, hydroxystearic, acetic, phthalic, terephthalic, butyric, m-nitrobenzoic, salicylic, phenylacetic, pyromellitic, p-phenylbenzoic, undecylenic, camphoric, furoic, acetamidobenzoic and o-aminobenzoic acids. Mercurio behenate is also useful, although the image formed with such material generally provides less contrast than do those produced with silver salt compositions.

As organic reducing agents there may be employed compounds such as protocatechuic acid, i.e., 3,4-dihydroxybenzoic acid; 2,3-dihydroxybenzoic acid; gallic acid; methyl gallate; spiroindane; 4-methoxy-1-hydroxydihydronaphthalene; 2,5-dihydroxybenzoic acid; hydroquinone; propyl gallate; pyrogallol; 4-azeloyl-bis-pyrogallol; 4-stearoyl pyrogallol; galloacetophenone; di-tertiary-butyl pyrogallol; gallic acid anilide; dodecyl gallate; ammonium gallate; ethyl protocatechuate; cetyl protocatechuate; l-hydroxy-Z-naphthoic acid; 2-hydroxy-3-naphthoic acid; phloroglucinol; catechol; 2,3-naphthalene diol; 4-lauroyl catechol; sodium gallate; protocatechualdehyde; 1f-methyl esculetin; 4,4dihydroxy biphenyl; 3,4-dihydroxyphenylacetic acid; 4(3,4-dihydroxyphenylazo) ben- Zoic acid; 2,2methylene-bis-3,4,5-trihydroxybenzoic acid; orthoand para-phenylene diamine; tetramethyl benzidine; 4,4,4diethylamino triphenylmethane; o-, m, and p-aminobenzoic acids; alphaand beta-napthols; tetrahydroquinoline. In general, such materials are cyclic-or preferably aromatic-organic compounds having an active hydrogen atom attached to an oxygen, nitrogen or carbon atom which is directly attached to an atom of the cyclic ring, the compound being a reducing agent for the noble metal ion of the heat-sensitive composition.

The ability of the reducing agent to cause the reduction of silver ion is readily shown by dissolving a few milligrams in a mixture of one volume of aqueous 10% silver nitrate solution in 4-5 volumes of ethyl `alcohol (or acetone or other organic solvent where necessary to dissolve the organic reagent). The prompt formation of a dark precipitate, either at room temperature or on gentle warming, demonstrates the required ability of the reagent to cause reduction of the silver ion.

Polystyrene is a preferred inert binder for the reactive components. Copolymers of styrene and isobutylene, which are equally moisture-resistant, are also effective. Less moisture-resistant binders, such as ethyl cellulose, may frequently be improved by the incorporation of small amounts of waxes or other moisture-proong materials; the free behenic acid of Example 1, While not essential to the operation of the heat-sensitive copy-sheet, is helpful in providing improved moisture-resistance as well as in lowering the conversion temperature. Overlying surface coatings may also be added, to protect the heatsensitive layer or for other purposes. One such structure adds to the sheet material, e.g. of Example l, a further coating of a thin layer of a mixture of three parts by weight of styrene-isobutylene copolymer, 30 parts of zinc oxide, and one part of silica gel, smoothly dispersed in and applied from heptane, the dried coating serving both to protect the heat-sensitive layer from moisture and abrasion and to serve as a contrasting white background for darkened image areas as viewed through the transparent paper support.

Pigments, fillers and other additives may likewise be incorporated in the heat-sensitive layer where desired, for example to alter the color of ythe background or converted image areas, Vto provide improved moisture-resistance, flexibility, or lm strength, to make the coating moderately radiation-absorptive, or for other purposes.

The appended drawing will serve to illustrate a simpliied form of the stabilized heat-sensitive copy-sheet of the invention, in which a backing of paper or the like is provided with a stabilized visibly heat-sensitive coating comprising a metal salt oxidizing agent, an organic reducing agent, and a perhalogenated aromatic acidic stabilizer.

What is claimed is as follows:

1. A stabilized heat-sensitive copy-sheet for making a clear and sharp reproduction of -a lgnaphic original by a thermographic copying procedure involving brief application of a heat-pattern corresponding to said original, said copy-sheet being visibly stable under normal storage conditions and when subjected to elevated temperatures up to about 610 C. for limited periods while being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 90-150 C. said copysheet including a visibly heat-sensitive layer containing, in intimate association, la normally solid organic acid salt of a noble metal, and a cyclic organic reducing agent for the noble metal ions, said reducing agent having an active hydrogen atom attached to an atom which is selected from the class of oxygen, nitrogen and carbon atoms and which is directly attached to an `atom of the cyclic ring, said reducing agent being capable of causing reduction of silver ion and precipitation of metallic silver on being dis-solved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, and said visibly heat-sensitive layer being characterized by the inclusion of 4a significant small amount of -a perhalogenated aromatic organic acidic stabilizer material suflicient to improve greatly the stability of the heat-sensitive layer.

2. The stabilized heat-sensitive copy-sheet of claim 1 in which the stabilizer material is a tetrachlorophthalic acidic material.

3. The stabilized heat-sensitive copy-sheet of claim 2 in which the acidic stabilizer material is tetrachlorophthalic anhydride.

4. For use in the preparation of a heat-sensitive copysheet, `a liquid coating composition consisting essentially of an inert volatile liquid vehicle, a film-forming binder, a normally solid organic acid salt of a noble metal, a cyclic organic reducing agent for the noble metal ions, and a perhalogenated aromatic organic acidic stabilizer material; said reducing agent having an active hydrogen atom attached to an atom selected from the class of oxygen, nitrogen and carbon atoms, said selected atom being directly attached to an atom of Ithe cyclic ring, said reducing agent being capable of causing reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent; and said coating composition being characterized by having a much greater resistance to degradation on standing at normal room temperature than an otherwise identical composition free of said perhalogenated aromatic organic acidic stabilizer material.

References Cited in the fle of this patent UNITED STATES PATENTS 

1. A STABILIZED HEAT-SENSITIVE COPY-SHEET FOR MAKING A CLEAR AND SHARP REPRODUCTION OF A GRAPHIC ORIGINAL BY A THERMOGRAPHIC COPYING PROCEDURE INVOLVING BRIEF APPLICATION OF A HEAT-PATTERN CORRESPONDING TO SAID ORIGINAL, SAID COPY-SHEET BEING VISIBLY STABLE UNDER NORMAL STORAGE CONDITIONS AND WHEN SUBJECTED TO ELEVATED TEMPERATURES UP TO ABOUT 60*C. FOR LIMITED PERIOD WHILE BEING RAPIDLY PERMANENTLY VISIBLY CHANGED ON HEATING TO A CONVERSION TEMPERATURE OF THE ORDER OF ABOUT 90-150*C. SAID COPYSHEET INCLUDING A VISIBLY HEAT-SENSITIVE LAYER CONTAINING, IN INTIMATE ASSOCIATION, A NORMALLY SOLID ORGANIC ACID SALT OF A NOBLE METAL, AND A CYCLIC ORGANIC REDUCING AGENT FOR THE NOBLE METAL IONS, SAID REDUCING AGENT HAVING AN ACTIVE HYDROGEN ATOM ATTACHED TO AN ATOM WHICH IS SELECTED FROM THE CLASS OF OXYGEN, NITROGEN AND CARBON ATOMS AND WHICH IS DIRECTLY ATTACHED TO AN ATOM OF THE CYCLIC RING, SAID REDUCING AGENT BEING CAPABLE OF CAUSING REDUCTION OF SILVER ION AND PRECIPITATION OF METALLIC SILVER ON BEING DISSOLVED AT MODERATE TEMPERATURE IN A SOLUTION OF AQUEOUS SILVER NITRATE IN AN ORGANIC SOLVENT, AND SAID VISIBLY HEAT-SENSITIVE LAYER BEING CHARACTERIZED BY THE INCLUSION OF A SIGNIFICANT SMALL AMOUNT OF A PERHALOGENATED AROMATIC ORGANIC ACIDIC STABILIZER MATERIAL SUFFICIENT TO IMPROVE GREATLY THE STABILITY OF THE HEAT-SENSITIVE LAYER. 